Makefile: kselftest: fix grammar typo

Correct typo in kselftest help text.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1395,7 +1395,7 @@ help:
 	@echo  '                    Build, install, and boot kernel before'
 	@echo  '                    running kselftest on it'
 	@echo  '  kselftest-clean - Remove all generated kselftest files'
-	@echo  '  kselftest-merge - Merge all the config dependencies of kselftest to existed'
+	@echo  '  kselftest-merge - Merge all the config dependencies of kselftest to existing'
 	@echo  '                    .config.'
 	@echo  ''
 	@echo 'Userspace tools targets:'